Magnesium hydroxide particles have been known for a long time and are used for medical, industrial and agricultural products in a wide variety of fields. For example, the medical products include an antacid and a lapactic and the industrial products include a flame retardant, soot desulfurizer, waste water neutralizer, ceramic raw material and sintering aid. The agricultural products include a magnesium fertilizer which neutralizes acid soil or supplies minerals.
A typical method of manufacturing the magnesium hydroxide particles is a seawater method in which seawater and slaked lime are reacted with each other. However, as about 0.1 g/L of a CO3 ion in terms of CO2 is contained in the seawater, when slaked lime is added without a pretreatment, CaCO3 is formed and contained in the particles as an impurity. Then, seawater is generally decarbonized before it is used in a reaction. Other methods of manufacturing the magnesium hydroxide particles include one in which bittern as an Mg source and caustic soda as an alkali source are reacted with each other, one in which MgO is neutralized, and one in which a magnesium salt and ammonia are reacted with each other to crystallize magnesium hydroxide. In these prior art methods of manufacturing magnesium hydroxide, the CO3 ion which forms an impurity such as CaCO3 or MgCO3 is undesired and tends to be excluded from a reaction system as much as possible.
Meanwhile, magnesium carbonate is manufactured by a soda ash method in which a magnesium salt solution and sodium carbonate are reacted with each other, an ammonium carbonate method in which a magnesium salt solution and an ammonium carbonate solution are reacted with each other, and a carbonic dioxide gas method in which a carbonic dioxide gas is caused to act on magnesium hydroxide. Since these methods are aimed to obtain magnesium carbonate, a large amount of the CO3 ion is injected and the final product is represented by the chemical formula (3-5)MgCO3.Mg(OH)2.(3-7)H2O, shows an X-ray diffraction image specific to basic magnesium carbonate, which differs from that of magnesium hydroxide, and has a BET specific surface area of not more than 80 m2/g.
Patent document 1 discloses a method of manufacturing a magnesium hydroxide particle having a large specific surface area by heating magnesium hydroxide and amorphous silicic acid in an aqueous medium to form magnesium silicate on the surface of a magnesium hydroxide particle. However, the reason that the specific surface area of the magnesium hydroxide particle obtained by this manufacturing method is large is assumed to be the influence of both magnesium silicate formed only on the surface and amorphous silicic acid which seems to remain unreacted partially. It is hard to say that the specific surface area of the magnesium hydroxide particle becomes large.
(patent document 1) JP-A 2003-40616